The 3MG Trial

16 Jun

Didn’t even know this trial was being done till Simon Carley posted on it. It came just too late to be added to be talk on the Crashing Asthmatic but there you go.

Here’s my two cents on the trial

Goodacre, Steve, Judith Cohen, Mike Bradburn, Alasdair Gray, Jonathan Benger, and Timothy Coats. “Intravenous or Nebulised Magnesium Sulphate Versus Standard Therapy for Severe Acute Asthma (3Mg Trial): a Double-Blind, Randomised Controlled Trial.” The Lancet Respiratory Medicine 1, no. 4 (June 2013): 293–300. doi:10.1016/S2213-2600(13)70070-5.


  • UK publicly funded
  • 34 EDs involved in the UK
  • acute severe asthma (remember it’s not that hard to get into that category)
  • specifically excluded those with life threatening features
  • centrally randomised and reasonable description of blinding
  • each pack had nebs and an infusion
  • IV Mag (2g) v 3 Mg Nebs v placebo
  • all dummys so it looked like everyone gets the same
  • treated as per SIGN and BTS guidelines, though I didn’t see details of how many salbutamol nebs they got etc…
  • two primary outcomes
    • hospital admission either at the time or within 7 days or
    • a change in breathlessness on a VAS


  • Big trial – 1000 folk, average age 35 or so. This is important as a trial this size is unlikely to be equaled.
  • about 75% admitted, no one died in the ED. 2 died overall (not sure why)
  • powered to detect a 10% difference in admission rates and there was only about an 6% difference favouring IV Mag. This didn’t reach stat significance of course.
  • Neb Mag didn’t seem to do much of anything
  • 7 of 1000 needed intubation emphasising how “severe” acute severe asthma is
  • Standard treatment with beta agonists and steroids was awesome as you can see in the placebo group.


  • to be honest this is hardly surprising. I’ve rarely, if ever, seen magnesium work like a magic drug. I’ve always given it cause it’s benign (which this trial reinforces) and there was some support for its use. That was enough for me. It’s hardly surprising that the trial was negative but I suspect the small improvement that is there is real but is just that – small. 
  • having never used nebulised Mag, I’m now not about to start.


I had some correspondence from the author of this paper [free full text] regarding the ‘atrial calming’ effect of magnesium. I certainly gave lots of IV magnesium for patients in fast AF. Usually not for the ‘lone AF’ patients, but the sickies with pneumonias and deranged electrolytes. I’d never considered that the supposed rate control effect of magnesium might be useful in the asthmatic to help control the slightly ridiculous tachycardia you get if you’re doing it right.

As mentioned, the paper is free and well worth a read.

2 Replies to “The 3MG Trial

  1. Pingback: The Crashing Asthmatic - Emergency Medicine Ireland

  2. Pingback: Management of life threatening asthma in the emergency department | First10EM

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